docs: def _check_panel(self, length):
          """
          Check that given fixed panel length evenly divides index.
  
          Parameters
          ----------
          length : int
              Fixed length with which to subdivide index
          """
          n = len(self.index)
          if divmod(n, length)[1] != 0:
              raise ValueError("Panel length '%g' must evenly divide length of series '%g'"
                               % (length, n))
          if n == length:
              raise ValueError("Panel length '%g' cannot be length of series '%g'"
                               % (length, n))